fix(cli): make setup conflict error actionable

src/cli/program/register.setup.test.ts

@@ -157,7 +157,7 @@ describe("registerSetupCommand", () => {
     await runCli(["setup", ...args]);
 
     expect(runtime.error).toHaveBeenCalledWith(
-      `${flag} cannot be combined with onboarding options.`,
+      `${flag} cannot be combined with onboarding options. Rerun with either --message/--yes or onboarding options, not both.`,
     );
     expect(runtime.exit).toHaveBeenCalledWith(1);
     expect(readConfigFileSnapshotMock).not.toHaveBeenCalled();

src/cli/program/register.setup.ts

@@ -261,7 +261,7 @@ export function registerSetupCommand(program: Command): void {
           .map((option) => option.long ?? option.short ?? option.flags)
           .toSorted();
         defaultRuntime.error(
-          `${systemAgentFlags.join(", ")} cannot be combined with onboarding options.`,
+          `${systemAgentFlags.join(", ")} cannot be combined with onboarding options. Rerun with either --message/--yes or onboarding options, not both.`,
         );
         defaultRuntime.exit(1);
         return;